BEIRUT: A Syrian opposition meeting due to begin in Saudi Arabia on Tuesday marks the most serious effort yet to unify President Bashar al-Assad's fragmented enemies, a step seen as vital to peace talks sought by world powers but which has riled Iran.

While the outlook for the political track recently launched in Vienna appears bleak - international divisions over Assad persist and the war in Syria has escalated - the Riyadh meeting offers the prospect of forging a more united opposition better able to negotiate with the government.

Assad's opponents and the governments that back them see it as a long-overdue step towards ending the disunity that has obstructed diplomacy: the Turkish-based political opposition that led the last round of failed peace talks two years ago was widely criticised as out of touch with forces on the ground.

The Riyadh meeting is meant to bring rebels, or those who represent them, to the table when negotiations begin.

Yet Iran, whose Shi'ite-led government is an arch rival of the Sunni kingdom of Saudi Arabia, has said the meeting is aimed at harming the Vienna peace talks and would cause their failure. The Vienna process envisages formal talks between the Syrian government and the opposition by Jan. 1.

The participants invited to Riyadh include powerful Islamist factions Islam Army and Ahrar al-Sham - a group whose founders had links to al Qaeda. Ahrar al-Sham still fights alongside the Nusra Front, al Qaeda's Syrian wing, while espousing a nationalist agenda.

Islam Army said in a statement that its commander, Zahran Alloush, would not attend because the group had lost control of the road he had been planning to use to exit the area in the Damascus suburbs where the group is based. Members of Islam Army's political office will attend instead, it said.

A dozen Free Syrian Army rebel groups will also attend, including groups vetted by the United States that have received foreign military aid. They include recipients of U.S.-made anti-tank missiles supplied to rebels in larger quantities since Russia intervened militarily on Assad's side on Sept. 30.

"It is the first time there is a meeting in Saudi - a meeting of soldiers and politicians - and it has a greater chance of success because Saudi is hosting it," said the head of one of the FSA groups.

"Saudi is a pivotal state in the region and for it to take this step - to host a conference of the Syrian opposition factions - certainly something real will result from it."

He declined to be named because of political sensitivities surrounding the conference. Underlining the complexities, media access to the conference is expected to be highly restricted.

An initial list of 65 invitees has grown to many more than that, sources familiar with arrangements say.

KURDS NOT INVITED, SCOPE FOR FRICTION SEEN

Even with backing from Saudi Arabia, the United States and others, analysts still see scope for friction in Riyadh, notably between groups battling to unseat Assad and members of the Damascus-based opposition who are expected to attend.

"Trying to get those two poles to agree to the same platform and to negotiate as one unit is going to prove very difficult if not impossible, it's not entirely clear to me whether it is even advisable," said Noah Bonsey, a senior analyst with the International Crisis Group.

A Syrian Kurdish group that has taken over wide areas of northern Syria and which has fought Islamic State with U.S. help meanwhile says it has not been invited to the talks at all. One of the Kurdish-run enclaves in Syria declared the Saudi talks "doomed to fail" without Kurdish involvement.

Diplomacy towards ending the nearly five-year conflict has accelerated since Russia deployed its air force to mount strikes in support of the Syrian army.

Iran has also intervened with more forces since then. Hundreds of its elite Revolutionary Guard Corps are fighting alongside the Syrian army in ground offensives being waged with Russian air cover in several areas of western Syria.

Saudi Arabia has in turn boosted support to the rebels. Saudi Foreign Minister Adel al-Jubeir said on Nov. 26 that a military option in Syria was still viable and support for the opposition fighting to topple Assad would continue.

Russia's air campaign has mostly hit areas of western Syria that are crucial to Assad's survival and where the Islamic State group - the stated target of the Russian intervention - has little or no presence.

All the rebels attending the meeting are enemies of Islamic State, which controls swathes of eastern Syria seized mostly from other insurgent groups. The Nusra Front, listed as a terrorist group by the United States, has also not been invited.

A diplomat who tracks Syria said Russia would object to the participation of both Islam Army and Ahrar al-Sham due its links to the Nusra Front and al Qaeda. "The Iranians feel uncomfortable with the Saudis leading," the diplomat added.

"The aim of agreeing the parameters of negotiations with the regime is hard to achieve."

But the spokesman for one of the FSA groups due to attend said Ahrar al-Sham were part of the Syrian people.

"We did not take part before in conferences before, but we hope that this conference will be important for the future of Syria and ending the war," said Abu Ghaith al-Shami of Alwiyat Seif al-Sham, which operates in southern Syria.